Eperimental studies on the structure of the lower mantle and themantle fractionation

在26 GPA和1600 ℃下测定了Mg_2SiO_4-Fe_2SiO_4系统中橄榄石（γ）-尖晶石（γ）-后尖晶石相平衡。结果表明，镁尖晶石（Fe/Mg+Fe<0.2）在极窄的压力区间内（1600 ℃时<0.15 GPa）分解为钙钛矿和镁橄榄石。离解压与铁含量几乎无关，与650 km处的离解压一致。这些结果表明，650 km不连续的尖锐性可能确实是由于这种分离的橄榄岩地幔。将尖晶石分解的相边界与反射波研究所暗示的尖锐不连续相结合，估计过渡带底部（655 km深度）的温度约为1600 ℃。考虑到在CaSiO_3-MgSiO_3-Al_2O_3系统和地幔橄榄岩中证实的主解离的解离，建立了下地幔的矿物学模型 关于我们 在2.5GPa和2000-3000 ℃的温度下对肥沃橄榄岩进行了试验。在下地幔条件下，富MgSiO_3钙钛矿为液相。以Mg/Si、Al/Si、Ca/Si和Ca/Al比值为约束条件，证明原始上地幔成分可以通过少量MgSiO_3-钙钛矿分馏（10-20wt%）从地幔源区获得。其结构特征为SiO_6八面体的三维骨架，Mg处于八配位的立方八面体中，晶胞压缩主要由SiO_6八面体的倾斜控制。利用布里渊散射光谱首次在常压和室温下测量了MgSiO_3钙钛矿单晶的弹性模量。我们的弹性数据与适用于下地幔的地震模型的比较表明，只有当MgSiO_3钙钛矿的剪切模量具有很强的负温度导数时，具有地幔岩或辉石化学计量比的岩石学模型才与实验数据相容。少

The olivine( )-modified spinel( )-spinel(y)-postspinel phase equilibria in the system Mg_2SiO_4-Fe_2SiO_4 were determined up to 26 GPA and 1600 C. It was demonstrated that the magenesian spinel (with Fe/Mg+Fe<0.2) dissociates into perovskite and magnesiowustite within an extremely narrow pressure interval (<0,15 GPa at 1600 C). The dissociation pressure is almost independent of iron content and coincide to that at 650km. These results indicate that the sharpness of the 650km discontinuity may indeed be due to this dissociation in peridotitic mantle. Assigning the phase boundaries of the spinel dissociation to the sharp discontinuity implied by study of reflected waves, the temperature at the base of the transition zone (655km depth) is estimated to be about 1600 C. Taking the dissociation of the majorite dissociation confirmed in the system CaSiO_3-MgSiO_3-Al_2O_3 and mantle peridotites into consideration, the mineralogical model was constructed down to the lower mantle.Melting experim … More ents on a fertile peridotite was performed up to 2.5 GPa and at temperatures 2000-3000 C. The MgSiO_3-rich perovskite was found to be the liquidus phase at the lower mantle conditions. Using the Mg/Si, Al/Si, Ca/Si, and Ca/Al ratio as constraints, it was shown that primitive upper mantle compositions can be derived from a chondritic source through a small degree of MgSiO_3-perovskite fractionation (10-20wt%).Crystal structure of MgSiO_3perovskite was determined at high pressures up to 9.6GPa as well as at atmospheric pressure, using single crystals. The structure was characterized by the three dimensional framework of SiO_6 octahedra with Mg in the eight-fold coordinated cuboctahedron and the unit cell compression is controlled dominantly by the tilting of SiO_6 octahedra. The singly-crystal elastic module of MgSiO_3 perovskite was determined for the first time under atmospheric pressure and room temperature using Brillouin spectroscopy. Comparison of our elasticity data with seismic models appropriste to the lower mantle reveals that petrological models having either pyrolite or pyroxene stoichiometries are compatible with the experimental data only if the shear modulus of MgSiO_3 perovskite has a very strong negative temperature derivative. Less